<!DOCTYPE html>
<html>
<head>
  <style>
  div,span {
    width: 100px;
    height: 40px;
    float:left;
    padding: 10px;
    margin: 10px;
    background-color: #EEEEEE;
  }
  </style>
  <script src="http://code.jquery.com/jquery-latest.js"></script>
</head>
<body>
  <div class="notMe">div class="notMe"</div>

  <div class="myClass">div class="myClass"</div>
  <span class="myClass">span class="myClass"</span>
<script>
    $(".myClass").css("border","3px solid red");
    $("span.myClass").css("border","4px solid red");
</script>

</body>
</html>